If src != nil, ParseFile parses the source from src and the filename
is only used when recording position information.
The type of the argument for the src parameter must be string,
[]byte, or io.Reader.
If src == nil, ParseFile parses the file specified by filename.

A LoadStmt loads another module and binds names from it:
load(Module, "x", y="foo").

The AST is slightly unfaithful to the concrete syntax here because
Skylark's load statement, so that it can be implemented in Python,
binds some names (like y above) with an identifier and some (like x)
without. For consistency we create fake identifiers for all the
strings.